Thin film sol-gel derived glass

ABSTRACT

Thermally-assisted organometallic sol-gel derived glasses have been found to permit fabrication of thin films sufficiently thin for telecom components. Inclusion of a photosensitizer in the film permits light of controlled intensity to modify refractive indices in the film to form useful structures.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/574,841, filed May 19, 2000, which is hereby incorporated byreference in its entirety.

This application is also related to a companion application Ser. No.09/574,840, filed on May 19, 2000, and assigned to the assignee of thepresent application.

FIELD OF INVENTION

This invention relates to a process for producing photosensitive thinfilms of sol-gel derived glass and to such films of a thickness usefulfor integrated optic devices produced thereby.

BACKGROUND OF THE INVENTION

The doctoral thesis by the application herein entitled “Photolithographyof Integrated Optic Devices in Porous Glass,” City University of NewYork, 1992, describes an organometallic system of inclusions in athermally-assisted, porous glass bulk material. The process forfabricating the glass requires introduction of a photosensitizer,exposure to light through a mask and two heat treatments. The doctoralthesis states that sol-gel techniques can be used to make the porousglass bulk material.

BRIEF DESCRIPTION OF THE INVENTION

The invention is based on the realization that the porous glasstechniques for bulk materials using thermally assisted, organometallic,sol-gel derived glass can be extended to thin films suitable for telecomcomponents and virtually free of lateral shrinkage. Consequently, avariety of highly desirable integrated optic components can be made bysuch a technique. Specifically, a technique for the photolithographicfabrication of integrated optic structures in thin films ofphotosensitive sol-gel glasses is described here. This techniqueinvolves the formation of a photosensitive sol-gel thin film includingan organometallic photosensitizer, on a suitable substrate (glass,silicon, or any other support material). Next, the photosensitive filmis exposed to white or ultraviolet light inducing a photochemicalreaction in the photosensitive sol-gel glass network with the endphotoproduct being a metal oxide. The photodeposited metal oxide ispermanently bound to the sol-gel film glass network as a glass modifierduring a heat treatment process, which in turn induces a permanentrefractive index increase in the glass. The refractive index increase isdependent on the concentration of the photosensitizer and on the lightenergy used in the exposure process. Therefore, a spatially varyinglight intensity during exposure results in a spatially varyingrefractive index profile. This refractive index profile induced in thefilm can be designed to guide light.

Exposure of the photosensitive sol-gel film to white or ultravioletlight induces the unbinding of the metal from the photoliabile moietycomponent of the photosensitizer followed by the binding of the metal tothe sol-gel film. The exposed regions of the sol-gel film are convertedto a metal oxide silica film by first and second step heatings at a lowtemperature and high temperature, respectively. The low temperaturedrives out the unexposed (unbound) photosensitizer and the unboundphotolabile moiety. The higher temperature step unbinds the organiccomponent from the bound photosensitizer and drives it off. This stepalso permanently binds the metal to the silica film forming a metaloxide glass modifier. If the sol-gel film is deposited on a glass orsilicon substrate, a metal oxide doped silica region of Si—O-M-O—Si isformed in the exposed regions acting as a glass modifier which in turnmodifies the refractive index. The unexposed photosensitizer is drivenoff during the heat treatment steps. Since no material is removed fromthe sol-gel film in this process, as in the case of prior-art processes,the resulting top surface is planar, thus leading to a simpler processfor producing devices and for achieving increased lifetime of resultingdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 4 and 5 are schematic side views of thin films in accordancewith the principles of this invention; and FIG. 3 is a block diagram ofthe steps for fabricating a structured thin film in accordance with thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a side view of film 11 of a sol-gel film with R-M-Xconstituents dissolved therein. The film is shown formed (usually by awell known spinning technique) on the SiO₂ surface layer 12 on a siliconsubstrate 13. The R constituents are taken from a class of volatileorganic materials consisting of CH₃, CH₃—CH₂, CH₃—CH₂—CH₂, the Mconstituents (metals) are taken from the class of metals consisting ofgroup IVB metals Ge, Sn and Pb, Group VIB including Se and Te, GroupVIIIA including Fe, Co, Ni, and Group IVA including Ti and Zr and rareearth metals, and the X constituents (photolabile moiety) are taken fromthe class consisting of chlorine, iodine, fluorine, bromine, andcarbonyls.

FIG. 2 shows an alternate embodiment where the sol-gel film 20 is formedon a glass substrate 21. FIGS. 1 and 2 represent the initial sol-gelsolution formed on appropriate substrates of silicon (FIG. 1) and glass(FIG. 2). The process of forming the sol-gel solution into useful filmstructures is discussed in connection with FIG. 3.

Specifically, FIG. 3 is a block diagram of the process for fabricatingstructured films from the sol-gel solution of FIGS. 1 and 2. Block 31 ofFIG. 3 represents the step of forming a sol-gel film with inclusions ofR-M-X on a suitable substrate (as shown in FIG. 1 or FIG. 2). Block 32represents the exposure of the film through a mask to light in a rangeof wavelengths from ultraviolet (UV) through the visible range. Thisstep unbinds the photolabile moiety (X) and binds the metal (M) to thesilicon oxide.

Block 33 of FIG. 3 represents the step of heating the film to about 300degrees C. for a time to bind the metal permanently to the SiO₂. Thisstep also drives off the unexposed organometallic photosensitizer fromthe entire sol-gel layer and the unbound photolabile moiety (X) from theexposed portions of the sol-gel layer. Block 34 of FIG. 3 represents thefinal heating step to about 900 degrees C. for unbinding the organiccomponent (R) from the bound photosensitizer and driving off thatcomponent. This step also permanently binds the metal to the silicasol-gel film forming a metal oxide glass modifier.

FIG. 4 shows the structure of FIG. 1 with a mask 40 in place. Mask 40 isopaque to the incident light (arrow 41) in regions 42 and 43 and istransparent to light in region 44. The result of exposure to light is astructured film (in excess of 1 micron) where the exposed region of thefilm includes Si—O-M-O—Si and the unexposed regions include SiO₂.

The concentration of photodeposited metal oxide determines the index ofrefraction of the exposed region which can be made relatively highcompared to that of adjacent regions. If we visualize region 44extending away from the view as indicated by the broken lines in FIG. 5,the resulting structure can be seen to represent a waveguide with the“core” being buried as indicated.

In one specific embodiment, a sol-gel film 1-10 microns thick was formedon a silicon substrate 1 cm×0.5 cm×0.1 cm thick with a SiO₂ surfacelayer<2 microns thick thereon. The sol-gel film included Sn (M) 2%, 1(X) 2%, and (CH₃)₃ (R) 2%. Region 44 has a width of 10 microns, exposedto light with a wavelength of 254 nm for 30 minutes. The exposed regionhad an index of refraction of 1.55 and the unexposed regions had indicesof refraction of 1.45. The film has a thickness of 1-10 microns afterprocessing and has unchanged lateral dimensions.

In another embodiment, a sol-gel film 1-10 microns thick was formed on aglass substrate 1 cm×0.5 cm×0.1 cm thick. The sol-gel film included Ti(M) 2%, Cl (X) 4%, and Cp (R) 4% where Cp is cyclopentadienyl. Region 44has a width of 10 microns, exposed to light with a wavelength of 514 nmfor 120 minutes. The exposed region had an index of refraction of 1.75and the unexposed region had indices of refraction of 1.45. The film hadan final thickness of 1-10 microns with the lateral dimensions thereofbeing unchanged.

1. A glass thin film derived from a thin film of photosensitive sol-geldoped with an organometallic photosensitizer, said organometallicphotosensitizer having a formula R-M-X, where X is a photolabile moiety,M is a metal, and R is a volatile organic compound, wherein said thinfilm of photosensitive sol-gel has a thickness in excess of one micron.2. A thin film as in claim 1, said film being formed on a substratehaving a surface including silicon and oxygen.
 3. A thin film as inclaim 2 wherein said surface comprises SiO₂ and is a silica enrichedthin layer on a silicon substrate.
 4. A thin film as in claim 1, saidfilm being formed on a glass substrate.
 5. A thin film as in claim 1,wherein R is taken from a class of low-volatile organic moleculesconsisting of CH₃, CH₃—CH₂, CH₃—CH₂—CH₂, and Cp, M is a metal taken froma class consisting of metals in Groups IVA, IVB, and VIB, transitionmetals and rare earth metals, and X is a photolabile moiety taken from aclass consisting of halogens and carbonyls.
 6. A thin film as in claim 3where R comprises CH₃, M comprises Sn, and X comprises I.
 7. A thin filmas in claim 3 wherein R comprises cyclopentadienyl.
 8. A thin film as inclaim 3 wherein M comprises Ti.
 9. A thin film as in claim 3 wherein Xcomprises Cl.
 10. A thin film as in claim 4 wherein R comprises CH₃. 11.A thin film as in claim 4 wherein M comprises Pb.
 12. A thin film as inclaim 4 wherein X comprises Cl.
 13. A thin film as in claim 2 includingthereon a mask opaque to light in the UV and visible ranges.
 14. A thinfilm of sol-gel derived glass on a silica substrate, said film includingat least one metal oxide doped silica region of Si—O-M-O—Si withadjacent regions of SiO₂, where M is a metal, said film having athickness substantially in excess of one micron and being free of cracksand lateral shrinkage, and wherein said metal oxide is photodepositedfrom an organometallic photosensitizer included in the sol-gel used toform said film.
 15. A method for forming a photosensitive sol-gel filmincluding regions of different indices of refraction, said methodcomprising the steps of forming a photosensitive sol-gel film includingan organometallic photosensitizer on a silica substrate, exposing saidfilm through a mask to light of a wavelength and for a time forunbinding different amounts of metal constituents and of said sensitizerin different sections along at least a first channel thereof, exposingsaid film to heat at a first temperature and for a time to drive off theunbound sensitizer and to bind the metal constituents of said sol-gelfilm, and exposing said layer to heat at a second temperature higherthan said first temperature for a time to unbind and drive off theorganic constituents of said sol-gel film.